Apparatus, systems, and methods to address evaporative cooling and wet compression for engine thermal management

ABSTRACT

An apparatus including a reciprocating internal combustion engine with at least one piston and cylinder set and an intake stream; at least one liquid atomizer in fluid communication with the intake stream operable to provide a plurality of liquid droplets with a diameter less than 5 μm to the intake stream; and a controller where the controller is able to adjust an index of compression for the engine by: calculating a wet compression level in response to an engine operating limit and adjusting the at least one liquid atomizer in response to the wet compression level.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/825,516, filed on Jun. 29, 2010, which claims the benefit ofU.S. Provisional Patent Application No. 61/269,844 filed Jun. 30, 2009,both of which are hereby incorporated by reference in their entirety.

BACKGROUND

The present application relates to thermal management of an internalcombustion engine, and more particularly, but not exclusively toevaporative cooling and wet compression of an internal combustionengine.

Engine knocking (also called detonation or spark knock and pinging)occurs in internal combustion engines when a portion of the air/fuelmixture in the cylinder does not combust with the rest of the air/fuelmixture in the cylinder at the precise time in the piston's stroke cycleas determined by the engine control system. The peripheral explosionscreate shock waves that can be heard by the operator. The effects of theengine knock shock waves can be of no consequence or catastrophic to theengine. In between these extremes, engine knock limits the compressionratio, ignition timing, and other engine operating parameters thateffect efficiency on reciprocating internal combustion engines. Withoutengine knock, these parameters could be changed to enable more efficientoperation and generally re-optimize the engine for superior performance.

Liquids can be used to cool combustion gases in internal combustionengines. The term wet compression refers to the act of vaporizing liquidduring compression. The phase change from liquid to vapor consumesrelatively large amounts of energy with a relatively small temperaturechange. Wet compression therefore allows a thermodynamic cycle thatefficiently compresses an air/liquid mixture with a lower temperatureincrease than compressing dry air, enabling higher compression ratio andpeak pressure at a constant peak temperature.

Humid air and water injection are related processes that provide somebenefits to reciprocating internal combustion engines. Humid air cycleshave been used in reciprocating engines and gas turbines to reduce NOxemissions. Water injection was used in WWII to increase aircraft enginepower density, primarily for takeoff, but there are differences thatallow wet compression to improve power density and efficiency relativeto humid air and water injection.

Wet compression may be confused with humid air thermodynamic cycles.Humid air cycles inject steam or other non-reactive vapor into theair/fuel stream. Vapor injection increases the thermal mass of theair/fuel mixture and dilutes the charge air. The dilution decreases thetendency to knock while maintaining stoichiometric combustion. Thelarger thermal mass reduces the peak temperatures in the combustionchamber therefore reducing NOx formation.

Because humid air is already in vapor phase when mixed with the air/fuelstream, it cannot go through a phase change during compression. Whenhumid air is compressed, the temperature increases almost as fast as ifthe air were dry. The temperature rise is slightly lower since thespecific heat ratio of the vapor, e.g. water, is often lower than thespecific heat ratio of the air/fuel mixture. Any improved knock marginis primarily dependant upon the dilution effect.

In water injection, large liquid droplets are added to an air/fuelstream to provide cooling from the phase change during compression andcombustion. Upon injection, water droplets may follow one of two paths:collision with internal surfaces or entrainment in the airflow.

Large water droplets may collide with internal surfaces during airflowdirection changes. The inertia of large water droplets can overwhelm thefriction from surrounding airflow and hinder the droplet's ability tofollow airflow changes around structures within the air ducting, intakemanifold, and cylinder, causing droplet collisions with internalsurfaces. If the surface is hot, such as in the cylinder, the liquid mayvaporize, thus making the system act similar to humid air injection. Ifthe droplet does not vaporize, it is pushed by the airflow along theinternal surface. As it is moving, the liquid may coalesce with otherliquid droplets. The ratio of surface area to volume decreases, whichreduces the effects of air temperature on liquid vaporization. Surfacewetting may also lead to engine corrosion issues such as cylinder linerscuffing and oil quality degradation.

If the droplets do not collide with a solid surface, they may remain inthe airflow. Eventually, at least some of the droplets may arrive in thecylinder. During compression, the air temperature surrounding thedroplets increases. Increased temperature causes heat to flow from theair to the water droplet. As the thermal gradient between the dropletand surrounding air increases, the heat transfer rate increases. Heattransfer across the large thermal gradients associated with largedroplets increases system entropy, however, thus reducing cycleefficiency.

Sufficiently large droplets have enough thermal inertia to remain inliquid form during combustion. Large droplets have a lower surface areato volume ratio than small droplets. Because heat transfer depends uponsurface area and heat required for total vaporization depends uponvolume, large droplets may not have enough time to completely vaporize.When the large droplets are present during combustion, they lower peakcylinder pressures and temperatures and slow flame propagation whichdecreases system efficiency.

While water injection and humid air injection may have some ability toreduce engine knock, various shortcomings persist: entropy produced fromlarge thermal gradients required during liquid vaporization; corrosionfrom surface wetting; and cooling the combustion process. Humid aircycles have temperature increases during compression equivalent to dryair compression limiting the efficiency of the combustion system. Thus,there is an ongoing demand for further contributions in this technicalarena.

SUMMARY

One embodiment of the present invention is a unique engine thermalmanagement system. Other embodiments include unique methods, systems,devices and apparatus involving managing intake air cooling, compressioninter-cooling and/or wet combustion with an engine thermal managementsystem. Further embodiments, forms, features, objects, aspects,benefits, and advantages of the present invention shall become apparentfrom the figures and descriptions provided herewith.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a system with an engine relating to oneembodiment.

FIG. 2 is a schematic diagram of a system with an engine relating toanother embodiment.

FIG. 3 is a schematic diagram of a system with an engine relating to yetanother embodiment.

FIG. 4 is a diagram illustrating an exemplary controller for enginethermal management.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is intended. Any alterations and further modifications in thedescribed embodiments, and any further applications of the principles ofthe invention as described herein are contemplated as would normallyoccur to one skilled in the art to which the invention relates.

One embodiment of the present application includes an apparatus andmethod for a reciprocating internal combustion engine for altering theisentropic or polytropic index of compression by adding a non-gas matterwith a sufficiently large surface area to maintain a low thermalgradient during the phase changes throughout intake and compressionstrokes. One specific example of this embodiment is the addition of finewater droplets (<5 micron) to the intake air stream where the dropletsact as an intercooler during the compression stroke, reducing peakcharge air temperature, and thus reducing the tendency for the engine toknock.

Wet compression is an engine conditioning process which utilizesdroplets that behave as though they are integral parts of the intake airwhile remaining in liquid form. For example, the droplets cool amajority of the intake air uniformly to avoid hot spots. For wetcompression conditions during air intake, smaller droplets maycontribute to improved dispersion in the charge air. For a similarvolume of liquid, the separation between droplets is less with smallerdiameter droplets, and correspondingly provides more cooling points andkeeps the bulk temperature closer to a saturation temperature. Whensmall droplets avoid contact with surfaces and substantially vaporizebefore the combustion process begins, the efficiency of the systemshould increase. These conditions may be met if the droplets aresufficiently small.

In another embodiment, entrained droplets in the intake air improve thevolumetric efficiency of a reciprocating internal combustion engine.Under normal conditions as reciprocating internal combustion enginesdraw air into the cylinder, the fresh air mixes with residual exhaustand typically is heated by the cylinder walls, piston, and head. Thisheating causes the gas to expand, thus reducing the amount of airintroduced to the cylinder in each cycle. Under wet compressionconditions, the entrained droplets keep the charge air cool, whichincreases the charge air density which may contribute to increased powerdensity.

Unfortunately, introducing liquid to the compression stage of an enginemay increase system entropy. Heat flows from the compressing air to theliquid to force the liquid droplets to vaporize. This heat transfer isdriven by a thermal gradient. Heat transfer across a thermal gradientincreases entropy, and the larger the thermal gradient, the more entropyis produced. In one embodiment a condition for liquid introduction tothe compression stage is a significant amount of liquid surface areatherefore there is a reduced thermal gradient utilized for the heattransfer. Liquid droplet diameter reductions generally increase thesurface area for a given amount of liquid. The reduced thermal gradientof the reduced diameter liquid droplets may provide a reduced entropylevel and result in improved efficiency of the combustion system.

Liquid that is present beyond the intake and compression stages andcontinues into the combustion stage may have ill effects. The thermalgradient tends to be large for the heat transfer, since the combustiontemperature is hot while the liquid is still at its saturationtemperature. Combustion temperatures may remain high before vaporizingthe water, allowing elevated NOx formation. If a similar amount of waterwere vaporized before combustion, it may improve NOx emissions andefficiency.

Further, when droplets carry into the combustion stage, the phasetransformation tends to cool the combustion process. Wet combustion maydecrease system efficiency due to a lower temperature and therefore adecreased pressure rise compared to dry combustion, as well as slowerflame propagation. Systems designed for intake air cooling orinter-cooling with wet compression may find it advantageous to avoidoperations that may create wet combustion conditions.

Understanding the parameters that affect droplet vaporizationcontributes to wet compression optimization. The energy consumed duringliquid vaporization drives the inter-cooling effects of wet compression.Droplet diameter is one parameter for vaporization properties. For agiven quantity of water for one embodiment, the time required fordroplet vaporization is partially dependant upon the diameter of thedroplet squared. At a constant thermal gradient relative to the bulk gastemperature, the time to vaporize a liquid droplet scales approximatelyquadratically with the droplet diameter. With a similar thermalgradient, a 5 μm droplet will likely vaporize in approximately 1/16^(th)the time of a 20 μm droplet and a 1 μm droplet will likely vaporize inapproximately 1/400^(th) the time because of the surface to volume ratioand dispersion effects of smaller droplets.

In one embodiment, the average droplet diameter is less than 10 microns.In another embodiment, droplet diameter is equal to or less than 5microns for droplets comprised of water. In yet another embodiment, itwas surprisingly discovered that a desirable inter-cooling effect may beachieved by keeping the maximum diameter of each droplet to less than2.7 microns in a 1200 RPM engine with the droplets comprised of water.

Water injection generally uses large droplets of water, 20 μm to over100 μm, and is intended to cool the combustion by vaporization andair/fuel dilution. Large droplets may provide limited cooling during thecompression stroke. In contrast to one embodiment of the presentapplication, wet compression uses small droplets, 1 μm or less to 5 μm,to allow faster vaporization, acting as an inter-cooler for thecompression stroke and more fully vaporizing before combustion. Smalldroplets generally are able to more fully vaporize before combustion andcontribute to reduced entropy production during vaporization. While theinter-cooling effect may be more efficient, the effect may also utilizeless water to provide a similar knock margin and NOx reduction. Lesswater is utilized since inter-cooling contributes to reduced compressionwork, therefore requiring less energy dissipation.

It should be appreciated that the fast vaporization rate of smalldroplets tends to reduce corrosion and oil-loss sometimes associatedwith water injection. Small droplets can potentially vaporize in lessthan a millisecond when exposed to large thermal gradients. Thus if asmall droplet is on a trajectory toward the cylinder wall, the heat fromthe cylinder will likely vaporize the droplet before it is able tocontact a cylinder surface. A larger droplet would take longer tovaporize, and may enable liquid-solid contact. It should be appreciatedthat small diameter liquid droplets are better able to follow airflowseffectively as compared to large vapor droplets, thus making portinjection schemes possible as an alternative to direct injection.

Typically, it is desired that droplets follow airflow to avoid wettinginternal surfaces. Larger droplets, common to water injection systems,are more likely to collide with engine surfaces. Surface wettingdecreases compression inter-cooling effectiveness and may causecorrosion issues. Droplets that collide with internal surfaces decreasetheir ratio of surface area to volume, which reduces the positiveeffects of wet compression. Liquids on internal surfaces may also becomea source for possible corrosion or lubricant removal. As the surfacearea to volume ratio of the droplets increases, so does the droplettrajectory's dependence upon the surrounding airflow.

As the droplet temperature increases from heat flow, the droplet reachesits saturation temperature and vaporizes. The heat used to create thisvaporization would otherwise be used to increase the temperature of thecompressing air, thus droplet vaporization may effectively reducecompression temperature, suppressing knock. During compression thepressure also increases, thus the saturation temperature increasesthroughout the compression stroke. Vaporization adds to the total numberof gas molecules in the compression chamber, thus it also adds to thetotal chamber pressure. This allows the isentropic or polytropic indexof compression for temperature to be lower than the effective isentropicor polytropic index of compression for pressure.

Entrainment and vaporization rate requirements suggest small, uniformwater droplets. Creating micron diameter droplets is difficult forinjectors using air or water pressure for atomization. One embodimentincludes a technology that can create droplets that are small anduniform which is referred to as ultrasonic atomization. Ultrasonicatomizers can produce water droplets with diameters lower than 1 μm.Ultrasonic atomizers effectively create microscopic waves that spraywater into the air stream. Droplet size depends upon driving frequency,which can be tightly controlled. The ability of an engine managementsystem to tightly control the droplet size may contribute to theoptimization of the engine operations.

Generally, certain applications utilizing ultrasonic atomizers may beplaced as closely as possible to the intake valves to reduce dropletagglomeration. One embodiment includes ultrasonic atomizers, followed bya large droplet separator apparatus, followed by an intake valve inclose communication. In one approach, the large droplet separator canact as the water/air/fuel mixer.

In other embodiments, additional techniques for droplet formation arepossible such as electrostatic atomization. While electrostaticatomization is capable of creating fine droplets, electrostaticatomization may not be desired in certain embodiments that include aspark ignited engine due to the electrical discharge that it maygenerate.

Another embodiment may include flash atomizers to provide similaratomization. Flash atomizers create small droplets by heatingpressurized liquid so that a portion boils when released from theatomizer to a lower pressure. The boiling action breaks an initial largedroplet into smaller droplets. This action may take place in acontinuous atomizer or heated injector (presumably common railinjector). Flash atomization also enables in-cylinder injection in areciprocating internal combustion engine, which can enhance performanceand controllability of the wet compression process. Flash atomizationmay also be accomplished by adding a gas that is highly miscible in theinjected coolant at high pressure but escapes as a vapor at lowpressures to avoid heating the liquid. For, example, carbon dioxidedissolves in water at high pressures, but causes secondary atomizationwhen released to lower pressures, even at ambient temperatures.

Flash atomization may be done by a steady-state atomizer, port injector,or direct in-cylinder injection (in order of increasing complexity andcontrol capabilities). For one embodiment, a common rail injector holdsfluid around 200 bar. At 200 bar liquid water can be heated to over 200°C. without vaporizing. Upon the injector opening, the pressurized hotwater is released to a lower pressure, e.g. 1 bar. At 1 bar, 200° C. isabove the boiling temperature for water. The liquid droplet atomizedfrom the injector begins to boil, including internal vaporization.Internal vaporization contributes to a primary droplet breaking intomany smaller droplets. Small secondary droplets are then available forwet compression applications.

Following atomization, injection may take place inside the combustionchamber or outside the combustion chamber. Within the combustionchamber, liquid can be injected during the intake stroke or just afterthe intake valve closes. Both internal injection and external injectionbefore the intake stroke improves volumetric efficiency by cooling theintake charge air, thus increasing its density. Internal injection afterthe intake valve closes provides greater cooling for a similar amount ofliquid injection due to a smaller air mass. Internal injection enablesinjection timing optimization that minimizes internal surface wetting byinjecting at a rate similar to the vaporization rate. Matching injectionand vaporization rates minimizes the residence time of water droplets inthe power cylinder. While internal injection provides greater control,injectors outside of the combustion chamber are generally lessexpensive.

It should be appreciated that any liquid may be flash atomized. In anillustrative embodiment, ethanol, or ethanol/water mixes, can beinjected as the primary fuel, with flash atomization increasing wetcompression, and correspondingly the upper efficiency limit, as well asimproved combustibility since more fuel is vaporized before combustion.This injection technique can also enhance engine efficiency withgasoline, though gasoline provides less cooling since it has arelatively lower enthalpy of vaporization. Alternatively, inert liquidssuch as water may be injected to provide cooling exclusively.

The source of the liquid coolant may be an issue as much as the choiceof coolant. Some liquid source contamination, such as carbon particlesin condensed exhaust gas vapors, may be benign to the engine, but maycause significant harm to injectors. Ultrasonic atomizers usually do notrely on small openings like pressurized injectors, and so they are lesslikely to plug due to contamination. Additionally, ultrasonic atomizersspray anything carried in the water supply, and so the water supplycontent should be considered.

In other embodiments, wet compression utilizes a source of engine-safewater. The water source may be met by three exemplary conditions:stationary primary power such as combined heat and power systems (CHP),exhaust gas recirculation (EGR), and exhaust water recovery systemssimilar to those used in fuel cells. CHP systems are generally connectedto water supplies, and adding components, such as water purifiers,generally will not significantly add to the system size. EGR coolingsystems provide for the uncontrolled introduction of water to the engineafter precipitation in the cooler. Alternatively, condensation from EGRcoolers may be collected and finely atomized to enable wet compression.

In one approach, cooled EGR water from an exhaust gas condenser mayserve as an adequate liquid source. Alternatively or additionally,external water may be used with filtering and/or distilling as desired.Because there is already significant heating and cooling taking place inengines, integrated distillation systems may be a cost effectiveapproach to providing clean water.

In another embodiment, the droplets removed from the air stream may becollected and returned to the entrainment system to attempt tore-introduce them at a reduced size. Droplet recycling applies in cooledEGR applications, for example, where large water droplets are present inthe air-fuel stream and may provide a majority of the water for theatomizer, which reduces the need for external water supplies. Water mayalso be recycled from the exhaust gas independent of other exhaustsystems such as an EGR cooler.

One approach to increasing effectiveness of wet compression includesremoving liquid droplets that are larger than desired. Dropletseparation increases the cycle efficiency by removing droplets thatwould otherwise cool the combustion process. Separation also increasesliquid use efficiency by enabling liquid recycling. For example, a 100micron diameter droplet that is captured and recycled can produce1,000,000-1 micron diameter droplets which would have 10,000 times thetotal surface area—significant for cooling.

Droplet separation may be done by several processes known in the art,for example, inertial separation or a screen. Inertial separation, whichuses the difference in mass vs. surface area between small and largeparticles, may take place in a cyclonic separator. Another inertialseparator uses barrier plates that the airflow is directed around.Either a cascading impactor or virtual impactor may be used for thistype of separation. Larger droplets may find it more difficult to followthe airflow around the barrier plates and impinge on the surfaces. Ascreen may provide barriers of known separation that don't allow largerdroplets through. Such a screen may have wire spacing less than 10microns. Another embodiment may utilize air filtration with certainadjustments relative to other standard intake pathway components. Thisembodiment may locate a droplet separator after the intercooler andthrottle.

In certain implementations, droplet separation may be placed after theliquid or particle injection into the gas stream to remove largeparticles that may cause ill effects when entering the cylinder of areciprocating internal combustion engine. Smaller droplets are allowedto pass through, enabling wet compression to take place. A majority oflarger particles are removed and may be recycled into smaller dropletsand entrained in the airflow again. Other separator methods may includegravitational or electrostatic forces to name a few.

Wet compression that takes place throughout the compression stroke mayover-cool the air/fuel mixture. A hybrid condition between wet and drycompression may provide optimal efficiency for a wide variety ofcompression ratios. A hybrid cycle begins with wet compression. Wetcompression at the beginning of the hybrid cycle minimizes entropyproduction during vaporization while reducing the temperature increaseduring compression. The appropriate amount of liquid is injected toforce a transition to dry compression during the compression strokeafter the liquid fully vaporizes. The hybrid cycle enables thepre-ignition temperature to be as high as possible without knocking. Theelevated temperature addresses the efficiency loss from full wetcompression. When the coolant completely vaporizes during compression,there is no further cooling during combustion and expansion, leaving amore optimal isentropic or polytropic index of compression. The index ofcompression may be isentropic for systems behaving as ideal gases orpolytropic for conditions when the ideal is not achieved.

The hybrid wet and dry compression cycle may deliver an exemplaryknock-free efficiency for compression ratios above the knock limit fordry compression. The optimal wet compression for a given pre-ignitiontemperature limit may be determined. The knock limited compression ratiomay be over 40:1, with over 70% theoretical efficiency possible. Thisknock-free compression ratio is possible due to the cooling effects ofwet compression, which limit the temperature and pressure rise that islikely to otherwise occur with dry compression.

In addition to improving efficiency, wet compression enables greaterengine control. The pre-ignition temperature may be controlled byadjusting the duration of wet compression. The duration of wetcompression may be set by the amount of liquid or solid coolant that isadded to the charge air. With a flash atomized injector in oneembodiment, liquid injection may vary from cycle to cycle, similar tocommon rail diesel injectors. The control of wet compression ratio mayenable rapid control of pre-ignition temperatures and pressures.

Pre-ignition temperature control provides many of the same benefits asvariable compression ratio engines. As the fuel quality, air-fuel ratio,or inlet air conditions change, the knocking characteristics of theengine change. Wet compression may enable the engine to adapt to thesechanges on a cycle by cycle basis. Homogeneous charge compressionignition (HCCI) is one application that requires control over air/fuelconditions at or near top dead center. Well controlled wet compressionmay enable the engine to adapt to auto-ignition changes due to fuel orcharge air changes. As charge air conditions change in one cycle, theinjector may adjust the amount of liquid injected in the next cycle tomaintain robust HCCI. Well controlled wet compression may provide thebenefits of a variable compression ratio engine, but with thepossibility of controlling and optimizing each engine cycle and cylinderseparately.

Because the air/fuel mixture is cooled during compression, the pressureincrease may be lower than it would be without cooling. Although thecompression ratio may be relatively high, the pressure ratio andtemperature ratio for the start of compression relative to the end ofcompression is lower than the temperature and pressure ratios for theexpansion stroke. This difference in pressure and temperature ratiosmakes the inter-cooling by wet compression operate in a manner similarto miller cycle engines, but with improved power density.

By introducing sufficiently small liquid droplets to the air-flow, theliquid becomes entrained and keeps the charge air cool during the intakestroke. The introduction of the small droplets may increase the totalvolumetric efficiency, and thus also increase the power density.Although the amount of liquid may be increased to induce wetcompression, small amounts of liquid will generally induce improvedvolumetric efficiency. Though reference is made to liquid introduction,phase change is believed to be the principle mechanism, thus solids mayalso be used.

At low levels of micro-droplet water introduction during the intakestage in one embodiment, the droplets often vaporize before reaching thecompression stage. Some droplets may keep the intake air cool as it isheated by the surrounding cylinder walls, piston, and cylinder head, aswell as the remaining exhaust leftover from the previous cycle. Byreducing the charge air temperature, more air is likely to enter thecylinder, which correspondingly increases the power density. Intake aircooling can improve performance of both compression ignition and sparkignition engines.

Engines generally require higher compression ratios or more advancedignition timing to increase efficiency. Increasing compression ratio andadvancing ignition timing generally increases NO_(x) emissions. Inanother embodiment, wet compression lowers pre-ignition temperature,therefore lowering peak temperature, and additionally lowering NO_(x)emissions while still providing high compression ratios and ignitiontiming in a thermodynamically efficient manner without the propensity toknock. Though this applies mainly to spark ignition engines, it alsoreduces NO_(x) production in compression ignition engines, even at highcompression ratios and advanced injection timing.

Although there is a potential to improve knock margin and reduce NOxproduction, uncontrolled introduction of liquid to an Otto or Dieselcycle engine may also increases cycle entropy, thus decreasing cycleefficiency. Overall system efficiency increases are made possible by thepresent application. Wet compression under optimized conditions mayallow advanced ignition timing, higher brake mean effective pressure(BMEP), lower combustion temperature, and higher compression ratios,each of which may contribute to system efficiency.

Wet compression is capable of reducing pre-combustion temperature andpressure. These pre-combustion effects may be beneficial for knockreduction but can be detrimental to efficiency. Excessive wetcompression may lead to significantly lower pre-combustion pressure,such that the combustion process provides a smaller peak pressure, andresults in lower efficiency. Wet compression may provide more coolingthan pressure reduction, so if the compression ratio is increased to theknock limit, the cycle efficiency will improve.

Engine knock generally limits the maximum boost pressure available in areciprocating internal combustion engine. This knock limit to boostpressure relation may be applied to engines with stoichiometric air/fuelmixtures. In one embodiment, wet compression suppresses knock byreducing the pre-ignition temperature of the air/fuel mixture. Thisknock suppression enables higher boost pressures, increasing the powerdensity and BMEP, which tends to increase system efficiency. In anotherembodiment, wet compression differs from water injection because theliquid is more completely vaporized before combustion, thus enabling anefficient combustion process and improved cycle efficiency.

Homogeneous charge compression ignition (HCCI) engines operate bycompressing the air/fuel mixture such that it auto-ignites near the endof the compression stroke. In such applications, the system operates totime the auto-ignition temperature, which if too early tends to causethe engine to knock heavily and if too late the engine may not fire atall. As the air/fuel mixture conditions change, the auto-ignitiontemperature also changes. Thus changing power output generally includesa change in the temperature reached at the end of the compressionstroke.

In yet another embodiment, wet compression may enable control over themaximum temperature reached during the compression stroke by providingcooling early during compression. In this embodiment, a compressionratio is set to provide HCCI during lean operation. As the air/fuelmixture becomes richer, the auto-ignition temperature is lowered,causing early ignition. Adding more fine water droplets can reduce thetemperature rise from compression and delay auto-ignition to theappropriate time. The logic to change the quantity of water added may bedependant upon sensors detecting the combustion conditions of theprevious cycle or cycles. In addition to HCCI control, the logic mayalso control incipient knock in an engine. When the engine powerrequirement is reduced, micro-droplet production may be reduced,reducing cooling and enabling auto-ignition at the appropriate time.

One aspect of wet compression is an inter-cooling effect duringcompression. An injected liquid quantity may be adjusted to have wetcompression only at the beginning of the engine cycle with most of thecompression stroke taking place with only gases. Because wet compressionmay reduce knock, enable higher boost pressures, increase compressionratio, advanced spark timing, etc., an operating condition may bedetermined by engine designers after removing knock limits from theirexisting optimization routines.

Insulating the combustion chamber in a reciprocating internal combustionengine is known to improve fuel conversion efficiency by retaining moreof the heat in the cylinder during expansion. Combustion chamberinsulation also provides hotter exhaust gases, which increases thepotential efficiency gained by combined heat and power systems andbottoming cycles such as turbo-compounding or Rankine cycles to improvetotal system efficiency. Conversely, the heat retained by the combustionchamber insulation after combustion contributes to knock in sparkignition engines and higher NO_(x) emissions in both Otto and Dieselcycle engines. Wet compression may alleviate negative effects ofcombustion chamber insulation for both Otto cycle and Diesel cycleengines.

Engine insulation has generally been limited to compression ignitionengines due to the knock issues in spark ignition engines. Research hasbeen done on adiabatic spark ignition engines, but no viable design hasresulted due to the knock limiting aspects. An embodiment of the presentinvention enables high compression ratio and boost for an insulatedspark ignition internal combustion engine.

Wet compression contributes to the reduction and/or elimination of theill effects of combustion chamber insulation by assisting to cool thecombustion chamber gases before compression. Evaporative cooling by wetcompression enables high compression ratios by reducing the index ofcompression, which allows lower temperature rises during compression.

In one embodiment, cylinder insulation may be more effective with wetcompression due to a larger relative effect. Additionally, the illeffects of cylinder/piston/head insulation, specifically increased knocktendency and NOx formation, may be reduced or eliminated (assuming othermechanisms such as but not limited to a three way catalyst is used instoichiometric combustion).

For one embodiment, wet compression contributes to lowering thein-cylinder temperature in a reciprocating internal combustion engine tosuppress knock and enabling combustion chamber insulation. Combustionchamber insulation may be an insulating coating of the piston, head, orcylinder where the insulating coating may be made of a ceramic. It alsoincludes creating engine components such as the piston, head, orcylinder out of insulating materials. Wet compression may also enableinsulation on diesel engines while maintaining low NO_(x) production andhigh volumetric efficiency.

Some alternative fuel designs are made more viable by one embodiment ofa system designed for wet compression. When bio-ethanol is producedunder certain conditions, the fuel is often separated from the waterthat was used in the biological processing. Initial separation may takeplace by distillation; final water/ethanol separation may take placewith rechargeable desiccants. Final separation adds cost and complexityto the ethanol production process. Making an engine run on water/ethanolmixes can reduce the processing requirements for ethanol production.Utilizing water/ethanol mixes as fuel could reduce the processingdemands, and thus fuel cost, for ethanol. Using current methods however,water/ethanol is difficult to reliably ignite due to the high latentheat of vaporization and the need for ethanol to be in gaseous form toignite.

Simultaneously, evaporative cooling and wet compression with pure wateris less efficient due to the surface tension of water complicating thecreation and vaporization of small droplets. Adding ethanol to watergenerally reduces the surface tension and boiling point of the water,simplifying atomization and the resulting smaller droplets.

One embodiment uses the high latent heat of ethanol/water mixes toprovide the benefits of evaporative cooling and wet compression byfinely atomizing the water/ethanol mix. This fine atomization (<5 μmdiameter) is made easier than with just water partly due to the surfacetension reducing qualities of ethanol in water. The benefits ofevaporative cooling and wet compression may enable higher compressionratios and reduced engine cooling (via insulated combustion chamber).The high compression ratio and insulated engine contribute to improvingefficiency and power density while reducing cold-starting problemsgenerally associated with ethanol/water mixes. In one embodiment of adirect injection engine, high compression ratio and insulated enginedesign may be used to provide open-throttle power management, furtherincreasing efficiency.

In one embodiment, un-throttled power output can be controlled by timingthe ethanol/water injection in the case of direct injection engines.Injection during the intake stroke may improve the evaporative coolingeffect during intake, and correspondingly increase the engine'svolumetric efficiency. Injection after the intake valve closes generallydoes not provide evaporative cooling during intake, thus a reducedamount of air may enter the power cylinder. Evaporative cooling and wetcompression effects may take place during compression to contribute tothe high compression ratio and efficiency.

Increasing the compression ratio too dramatically may cause catastrophicfailure upon disruption of the liquid delivery system—having thepotential to result in excessive knock. Other system optimizations maytake place which would not make water disruption catastrophic.

In one embodiment, small amounts of water introduced to the intake aircan benefit the engine in ways that are not completely dependant uponthe liquid delivery system. Wet compression allows: ignition timingadvances, higher knock-free boosting (reducing friction effects onefficiency), and/or reduced combustion temperatures (improving NOx andreducing heat transfer to the engine wall). These effects are presentwhen, in one embodiment, re-optimizing an engine utilizes wetcompression but does not catastrophically fail without it. The systemmay be designed such that if the water system fails, the engine may runsub-optimally with turbo-boost reductions and delayed ignition timing,but may not catastrophically fail.

FIG. 1 depicts a system 10 of one embodiment with an engine 12 which isof the reciprocating piston type having one or more reciprocatingpistons C. In one form, engine 12 is of the four stroke diesel-fueledtype with compression ignition and fuel injection. In other embodiments,engine 12 can be of a spark-ignited type, the two-stroke type, a rotarytype such as a wankel engine, and/or may not utilize any form of fuelinjection, to name just a few alternative possibilities. Furthermore,other embodiments may be differently fueled, such as by gasoline,ethanol, hydrogen, natural gas, propane, other gaseous fuels, and/or ahybrid combination of fuel types—just to mention some possibilities. Itis also contemplated that system 10 may, as an alternative to being usedto provide power to mobile applications such as vehicles, provide powerto stationary applications, such as electrical power generators, pumps,and the like. On such implementation is depicted as stationary system 10a, and more specifically depicted with electric power generator 12 abeing driven by engine 12—it being understood that both other stationaryand mobile applications (not depicted) are also contemplated to bewithin the scope of the present application. In addition, system 10 maybe used in hybrid applications that include one or more power sources inaddition to engine 12, such as batteries, fuel cells—to name a few.

Engine 12 includes an air intake stream 14. Air intake stream 14 is influid communication with a liquid atomizer 18. Liquid atomizer 18 may bean ultrasonic atomizer, flash atomizer, electrostatic atomizer or othersuitable type known to one skilled in the art. As air enters air intakestream 14, liquid atomizer 18 provides liquid droplets to the air. Asthe air with entrained liquid droplets continues through the air intakestream 14 of this embodiment, the liquid droplets are separated based onsize by flowing through a liquid droplet separator 16. Some embodimentsmay not include a separator as shown in FIG. 1 or separator 16 may beconfigured or positioned to be more integrated with engine 12 oratomizer 18. Liquid droplet separator 16 may be an inertial,gravitational or screen based separator. An inertial separator may becyclonic or transverse flow through parallel plates. After passingthrough liquid droplet separator 16, the air with entrained liquiddroplets enters engine 12 during an intake cycle.

A controller 20 is connected to and communicates with an atomizercontrol signal pathway 28 and various devices of engine 12 through a setof engine control signal pathways 26. Typically, controller 20 may beincluded in a standard type of Engine Control Module (ECM), includingone or more types of memory 22. Controller 20 can be an electroniccircuit comprised of one or more components, including digitalcircuitry, analog circuitry, or both. Controller 20 may be a softwareand/or firmware programmable type; a hardwired, dedicated state machine;or a combination of these.

In one embodiment, controller 20 is a programmable microcontrollersolid-state integrated circuit that integrally includes one or moreprocessing units 24 and memory 22. Memory 22 may be comprised of one ormore components and may be of any volatile or nonvolatile type,including the solid state variety, the optical media variety, themagnetic variety, a combination of these, or such different arrangementas would occur to those skilled in the art. Further, while only oneprocessing unit 24 is specifically shown, more than one such unit may beincluded. When multiple processing units 24 are present, controller 20may be arranged to distribute processing among such units, and/or toprovide for parallel or pipelined processing if desired. Controller 20functions in accordance with operating logic defined by programming,hardware, or a combination of these.

In one form, memory 22 stores programming instructions executed byprocessing unit 24 of controller 20 to embody at least a portion of thisoperating logic. Alternatively or additionally, memory 22 stores datathat is manipulated by the operating logic of controller 20. Controller20 may include signal conditioners, signal format converters (such asanalog-to-digital and digital-to-analog converters), limiters, clamps,filters, and the like as needed to perform various control andregulation operations described in the present application. Controller20 receives various inputs and generates various outputs to performvarious operations as described hereinafter in accordance with itsoperating logic.

FIG. 2 illustrates a further embodiment of a system 10 with an engine 12having one or more reciprocating pistons C positioned in a cylinder 13and an air stream 14. For each cylinder 13 of engine 12, a liquidatomizer 18 and a liquid droplet separator 16 are positioned in closeproximity to cylinder 13. One embodiment allows for injection of liquiddroplets into air stream 14 just before entering cylinder 13. Anotherembodiment allows for direct injection of liquid droplets atomized byliquid atomizer 18 and separated by liquid droplet separator 16 intocylinder 13. In a further embodiment, each liquid atomizer 18alternatively communicates directly with cylinder 13 without liquiddroplet separator 16. Each liquid atomizer 18 is in communication vialines 28 with controller 20 to allow optimization of engine operatingparameters via lines 26 on an individual basis based on wet compressionperformance in each cylinder 13.

FIG. 3 illustrates yet another embodiment where a system 10 includes anengine 12 with reciprocating pistons C similar to the embodiment shownin FIG. 1. Engine 12 includes an air intake stream 14 and an exhaust airstream 32. Air intake stream 14 is in fluid communication with a liquidatomizer 18 and a liquid droplet separator 16. Exhaust air stream 32 isin fluid communication with an EGR system 30. A water source 40 is incommunication with EGR system 30 to receive water via line 42 fromrecirculating exhaust gases. Water source 40 is in further communicationwith liquid atomizer 18 to supply water via line 44 to liquid atomizer18 for droplet formation. Water source 40 is also in communication withliquid droplet separator 16 to receive water via line 46 from largedroplets separated out of the air stream.

The index of compression whether isentropic or polytropic is adjustedwith wet compression resulting from the selective and controlledinjection of small water droplets in the air intake of a reciprocatingengine. In one embodiment, controller 20 may be responsible forcollecting engine operating parameters and establishing the relatedengine operation limits to induce efficient and optimized compressionand combustion conditions while limiting engine knock and NOx emissions.

Various engine operating parameters contribute to engine operatinglimits. Most reciprocating engines monitor fuel quality, air/fuel ratioand intake air temperature as each of these parameters may be a factorin engine performance. Further parameters such as charge airtemperature, charge air density and peak pressure can be used todetermine engine operating limits or used as an engine operating limitdirectly. Pre-ignition temperature can be used as an engine operatinglimit when controlling knock conditions. A knock detector may alsoprovide readings to be used as an engine operating limit. Additionally,a NO_(x) detector may provide readings for an engine operating limit.

Engine operating limits are determined to optimize engine performanceincluding eliminating engine knock, increasing volumetric efficiency andincreasing over all peak performance and efficiency. A wet compressionlevel can be calculated that allows control over the engine operatingparameters for each cycle of the piston and cylinder in the engine. Oneembodiment would include a wet compression injection schedule that canbe modified on a cycle by cycle basis.

Controller 20 may adjust additional engine operating parameters as thelogic performs a re-optimization module where the engine conditions arebased on the resulting performance with wet compression. In FIG. 4, anembodiment is shown with apparatus 400 which includes controller 420with various components illustrated as representative modules, inputsand outputs.

Engine operating limit module 405 is structure to determine operatingconditions in a number of modules which may be controlled when a wetcompression level is applied in response to the engine operating limit.

One embodiment is shown as Module 430 which is a pre-ignition conditionmodule 430 structured to determine engine operating limits including adesired temperature change based on a detected pre-ignition temperature432 and ignition parameters 434 selected for optimized engineperformance. The temperature change is used by wet compression module410 to determine a level of wet compression capable of accomplishing thetemperature change.

Another embodiment is shown as Module 440 which is an adaptiveperformance module structured to determine engine operating limits toallow dynamic response to changes in engine conditions such as, but notlimited to, fuel quality 442, air/fuel ratio 444 and intake or chargeair temperature 446. The engine operating limits are used by wetcompression module 410 to determine a level of wet compression capableof accomplishing the engine operating change.

Yet another embodiment is shown as Module 450 which is an HCCI module450 structured to determine engine operating limits for a homogeneouscharge compression ignition engine including an auto-ignition timingbased on auto-ignition conditions 452 of the last cycle or a function ofsome number of previous cycles selected to ensure optimal ignitionsequencing with piston position 454. The engine operating limits areused by wet compression module 410 to determine a level of wetcompression capable of accomplishing the auto-ignition timing.

Still another embodiment is shown as Module 460 which is a knockreduction module 460 structured to determine engine operating limitsbased on knock detection readings 465. The engine operating limits areused by wet compression module 410 to determine a level of wetcompression capable of accomplishing the knock reduction.

Module 410 is a wet compression level module 410 structured to determinea level of wet compression to achieve the engine operating limitsdetermined in module 405. One specific embodiment may includedetermining a wet compression level incorporating a hybrid compressionschedule 415.

Module 470 is an injection schedule module 410 structured to determinedan injection amount 472, an injection timing 474, and an injectionlocation 476 to provide the determined wet compression level from module410.

A further embodiment is shown as Module 480 which is a reoptimizationmodule 480 structured to determine engine performance parameters thatmay be re-optimized following the reduction of knock limits with wetcompression. Some of the performance parameters may include increasedboost pressure 482, increased compression ratio 484, and advanced sparktiming 486, to name a few.

Many further embodiments of the present application are envisioned.Among the inventions desired to be protected are:

One embodiment includes a reciprocating internal combustion engine withat least one piston and cylinder set and an intake stream; at least oneliquid atomizer in fluid communication with the intake stream operableto provide a plurality of liquid droplets with a diameter range lessthan 10 μm to the intake stream; and a controller wherein the controlleris operable to adjust an index of compression for the engine performedon an engine cycle by engine cycle basis by calculating a wetcompression level in response to an engine operating limit such as apre-ignition temperature threshold which may be determined in responseto a set of current engine knock detector readings; and adjusting the atleast one liquid atomizer in response to the wet compression level byselecting a liquid injection amount; and selecting a liquid injectiontiming.

A feature of the present embodiment includes at least one liquid dropletseparator in fluid communication with the intake stream positionedupstream from the at least one liquid atomizer and downstream from theengine. Another feature includes a diameter range of less than 2.7 μm.The wet compression level may include a feature where an amount ofliquid is provided for a transition to a dry compression state after theamount of injected liquid fully evaporates.

A further feature may include a plurality of liquid droplet injectors, aplurality of liquid droplet separators and a plurality of piston andcylinder sets where the liquid droplet injectors and the liquid dropletseparators are provided for each piston and cylinder set and where eachliquid droplet injector and each liquid droplet separator are in closeproximity to each piston and cylinder set. Yet a further feature mayinclude the engine operating limit being determined in response to a setof auto-ignition conditions and where calculating the wet compressionlevel is based on a set of engine cycle conditions changing with eachengine cycle.

Another embodiment includes operating a reciprocating internalcombustion engine with at least one piston and cylinder set and an airintake stream; atomizing a plurality of liquid droplets with a diameterless than 5 μm; providing the plurality of liquid droplets to the airintake stream; separating the plurality of liquid droplets; providingthe plurality of liquid droplets in the air intake stream to the engine;and optimizing an engine operating limit based on the plurality ofliquid droplets in the air intake stream including calculating a wetcompression level in response to the engine operating limit; andadjusting a liquid atomizer in response to the wet compression level byselecting a liquid injection amount; and selecting a liquid injectiontiming. Providing the plurality of liquid droplets in the air intakestream to the engine may include injecting the plurality of liquiddroplets into the air intake stream according to a liquid dropletinjector sequence.

Yet another embodiment includes a reciprocating internal combustionengine with at least one piston and cylinder set and an intake stream;at least one atomizer means for atomizing a coolant in fluidcommunication with the intake stream operable to provide a plurality ofcoolant particles with a diameter range less than 5 μm to the intakestream; and a control means operable to adjust an isentropic index ofcompression for the engine by a means for detecting a knock level of theengine; a means for calculating a wet compression level in response tothe knock level of the engine exceeding a knock threshold; and a meansfor adjusting a coolant atomization control schedule for the at leastone atomizer means in response to the wet compression level. Calculatingthe wet compression level may include determining an amount of coolantparticles when the coolant particles are provided on a continuous basis;and determining a duration for providing coolant particles when the atleast one means for atomizing a coolant is capable of providing a setamount of coolant particles to the intake stream.

A further inventive apparatus includes: a reciprocating internalcombustion engine with at least one piston and cylinder set and anintake stream; at least one liquid atomizer in fluid communication withthe intake stream operable to provide a plurality of liquid dropletswith a diameter range less than 10 μm to the intake stream; and acontroller wherein the controller is operable to adjust an index ofcompression for the engine by calculating a wet compression level inresponse to an engine operating limit; and adjusting the at least oneliquid atomizer in response to the wet compression level.

Features of the inventive apparatus include at least one liquid dropletseparator in fluid communication with the intake stream positioneddownstream from the at least one liquid atomizer and upstream from theengine; a diameter range of less than 5 μm or less than 2.7 μm; theplurality of liquid droplets including a fuel/water mix; adjusting theat least one liquid atomizer further including: selecting a liquidinjection location, selecting a liquid injection amount, and selecting aliquid injection timing; and a closed system water source in fluidcommunication with the at least one liquid atomizer.

Further features of the inventive apparatus may include the engineoperating limit having a pre-ignition temperature threshold where thepre-ignition temperature threshold may be determined in response to aset of engine knock detector readings and/or where the engine operatinglimit is determined in response to a set of auto-ignition conditions andwhere calculating the wet compression level is based on a set of enginecycle conditions changing with each engine cycle. The wet compressionlevel may include an amount of liquid determined to provide a transitionto a dry compression state after the amount of liquid fully evaporates.The inventive apparatus may further include a plurality of liquiddroplet injectors, a plurality of liquid droplet separators and aplurality of piston and cylinder sets where the liquid droplet injectorsand the liquid droplet separators may be provided for each piston andcylinder set and where each liquid droplet injector and each liquiddroplet separator may be in close proximity to each piston and cylinderset. Adjusting the index of compression may be performed on an enginecycle by engine cycle basis. The liquid atomizer may further include aprocess selected from the group consisting of ultrasonic atomization,electrostatic atomization, flash atomization and combinations thereof.

A further embodiment is a method including: operating a reciprocatinginternal combustion engine with at least one piston and cylinder set andan air intake stream; atomizing a plurality of liquid droplets with adiameter less than 5 μm; providing the plurality of liquid droplets tothe air intake stream; separating the plurality of liquid droplets;providing the plurality of liquid droplets in the air intake stream tothe engine; and optimizing an engine operating limit based on theplurality of liquid droplets in the air intake stream includingcalculating a wet compression level in response to the engine operatinglimit; and adjusting a liquid atomizer in response to the wetcompression level by selecting a liquid injection amount and selecting aliquid injection timing.

Further features of this method include providing the plurality ofliquid droplets in the air intake stream to the engine by injecting theplurality of liquid droplets into the air intake stream according to aliquid droplet injector sequence and the liquid droplets having adiameter range of less than 2.7 μm.

Yet a further inventive apparatus includes: a reciprocating internalcombustion engine with at least one piston and cylinder set and anintake stream; at least one atomizer means for atomizing a coolant influid communication with the intake stream operable to provide aplurality of coolant particles with a diameter range less than 5 μm tothe intake stream; and a control means operable to adjust an isentropicindex of compression for the engine by: a means for detecting a knocklevel of the engine; a means for calculating a wet compression level inresponse to the knock level of the engine exceeding a knock threshold;and a means for adjusting a coolant atomization control schedule for theat least one atomizer means in response to the wet compression level.Features of this further inventive apparatus include calculating the wetcompression level by determining an amount of coolant particles when thecoolant particles are provided on a continuous basis and determining aduration for providing coolant particles when the at least one means foratomizing a coolant is capable of providing a set amount of coolantparticles to the intake stream; and where the at least one piston andcylinder set at least partially include insulating material.

Any theory, mechanism of operation, proof, or finding stated herein ismeant to further enhance understanding of the present invention and isnot intended to make the present invention in any way dependent uponsuch theory, mechanism of operation, proof, or finding. It should beunderstood that while the use of the word preferable, preferably orpreferred in the description above indicates that the feature sodescribed may be more desirable, it nonetheless may not be necessary andembodiments lacking the same may be contemplated as within the scope ofthe invention, that scope being defined by the claims that follow. Inreading the claims it is intended that when words such as “a,” “an,” “atleast one,” “at least a portion” are used there is no intention to limitthe claim to only one item unless specifically stated to the contrary inthe claim. Further, when the language “at least a portion” and/or “aportion” is used the item may include a portion and/or the entire itemunless specifically stated to the contrary. While the invention has beenillustrated and described in detail in the drawings and foregoingdescription, the same is to be considered as illustrative and notrestrictive in character, it being understood that only the selectedembodiments have been shown and described and that all changes,modifications and equivalents that come within the spirit of theinvention as defined herein or claims that follow are desired to beprotected.

What is claimed:
 1. An apparatus comprising: an internal combustionengine with at least one piston and at least one cylinder and an intakestream; one or more liquid atomizers in fluid communication with theintake stream structured to provide a plurality of liquid droplets witha mean diameter less than 30 μm to the intake stream; a controllerstructured to calculate a wet compression level based on a reading froma NOx sensor and an engine operating limit, wherein the wet compressionlevel is effective to reduce NOx in an exhaust stream from the at leastone cylinder; wherein the one or more liquid atomizers are responsive tothe controller to adjust the wet compression level to reduce NOx in theexhaust stream, and wherein the wet compression level corresponds to anamount of liquid determined to force a transition to a dry compressionstate after the amount of liquid fully vaporizes during compression ofthe intake stream in the at least one cylinder.
 2. The apparatus ofclaim 1, further including at least one liquid droplet separator influid communication with the intake stream positioned downstream fromthe at least one of the liquid atomizers and upstream from the engine.3. The apparatus of claim 1, wherein the one or more liquid atomizersinclude means for providing a majority of the liquid droplets have amaximum diameter of less than 5 μm.
 4. The apparatus of claim 1, whereinthe one or more liquid atomizers include means for providing a majorityof the liquid droplets have a maximum diameter of less than 2.7 μm. 5.The apparatus of claim 1, further comprising: means for selecting aliquid injection location; means for selecting a liquid injectionamount; and means for selecting a liquid injection timing.
 6. Theapparatus of claim 1, further comprising: a closed water source in fluidcommunication with at least one of the liquid atomizers.
 7. Theapparatus of claim 1, wherein the engine operating limit includes apre-ignition temperature threshold and further comprising means fordetermining the pre-ignition temperature threshold as a function of oneor more engine knock detector readings.
 8. The apparatus of claim 1,wherein the wet compression level corresponds to an amount of liquiddetermined to provide a transition to a dry compression state after theamount of the liquid fully evaporates.
 9. The apparatus of claim 1,further comprising a plurality of liquid droplet injectors, a pluralityof liquid droplet separators and a plurality of piston and cylindersets; wherein the liquid droplet injectors and the liquid dropletseparators are provided for each of the plurality of piston and cylindersets; and wherein each of the plurality of liquid droplet injectors andeach of the plurality of liquid droplet separators are in closeproximity to each of the plurality of piston and cylinder sets.
 10. Theapparatus of claim 1, wherein the controller is structured to determinethe engine operating limit in response to a set of auto-ignitionconditions and to calculate the wet compression level based on a set ofengine cycle conditions changing with an engine cycle.
 11. The apparatusof claim 1, wherein the at least one of the liquid atomizers includesmeans for providing at least one of: ultrasonic atomization,electrostatic atomization, and flash atomization.
 12. A methodcomprising: operating a reciprocating internal combustion engine with atleast one piston and cylinder set and an air intake stream; generating aplurality of liquid droplets having a mean diameter less than 30 μm;providing the plurality of liquid droplets to the air intake stream ofthe engine; and controlling a wet compression level in to reduce NOx inan exhaust stream based on a reading from a NOx sensor by selecting atleast one of a liquid injection amount and a liquid injection timing,wherein the wet compression level corresponds to an amount of liquiddetermined to force a transition to a dry compression state after theamount of liquid fully vaporizes during compression of the intake streamin the at least one cylinder.
 13. The method of claim 12, whereinproviding the plurality of liquid droplets in the air intake stream tothe engine includes injecting the plurality of liquid droplets into theair intake stream according to a liquid droplet injector sequence. 14.The method of claim 12, wherein a majority of the liquid droplets have amaximum diameter of less than 5 μm.
 15. The method of claim 12, whereina majority of the liquid droplets have a maximum diameter of less than2.7 μm.
 16. The method of claim 12, further comprising: atomizing liquidto provide the plurality of liquid droplets.
 17. The method of claim 16,wherein the atomizing of the liquid includes at least one of: ultrasonicatomization, electrostatic atomization, and flash atomization.
 18. Themethod of claim 12, which includes separating the plurality of liquiddroplets from larger liquid droplets.
 19. The method of claim 12,wherein the controlling is performed as a function of one or more engineoperating conditions and includes regulating the wet compression level.20. An apparatus comprising: an internal combustion engine with at leastone reciprocating piston and at least one cylinder and an intake stream;means for atomizing a coolant in fluid communication with the intakestream operable to provide a plurality of coolant particles with amaximum diameter less than 30 μm to the intake stream; a controllerconfigured to adjust an index of compression for the engine by:detecting a NOx level of the engine; calculating a wet compression levelin response to the NOx level of the engine exceeding a NOx threshold;and adjusting a coolant atomization control schedule for the at leastone atomizer means in response to the wet compression level.
 21. Theapparatus of claim 20, wherein the controller is further configured to:determine an injection amount of coolant particles when the injectionamount of coolant particles are provided on a continuous basis; anddetermine a duration for providing the injection amount of coolantparticles when the at least one means for atomizing a coolant is capableof providing the injection amount of coolant particles to the intakestream.
 22. The apparatus of claim 20, wherein the at least onereciprocating piston and cylinder at least partially include aninsulating material.
 23. The apparatus of claim 20, wherein the wetcompression level corresponds to an amount of liquid determined to fullyvaporize throughout compression of the intake stream in the at least onecylinder.